U.S. Pat. Nos. 5,038,965 and 5,147,073 disclose fluid pump dispensers for pharmaceutical applications wherein fluids are discharged at a predetermined pressure and wherein a predetermined dosage is delivered regardless of the method of actuation. These dispensers have outlet ports through which the fluid is discharged and have inlet ports through which the fluid to be dispensed is fed into the dispensers from a container supplying the fluid. To ensure delivery of a predetermined dosage, a pharmaceutical dispenser is designed so that its exit port is held closed during actuation until a predetermined point on the down stroke is reached. At this point the outlet port is opened and the discharge of the predetermined dosage is initiated. The dispensers disclosed in the above identified patents employ an additional spring, other than the conventional return spring, which is used to hold the outlet port closed during part of the actuation. The elimination of the additional spring is most desirable, since such simplification decreases manufacturing difficulties, thus increasing performance and quality of the product.
The present invention is directed toward pharmaceutical dispensers which eliminate the additional spring and enable the outlet port to be held closed solely by the use of hydraulic forces, thus resulting in reduction of manufacturing difficulties and in enhanced performance.